THE SOL-GEL GROUP

Francoise Damay, Ph.D. (Resume)

"Transport Properties of NAFION^TM / SiO₂-P₂O₅-ZrO₂ Composite Membranes for Proton Exchange Membranes Fuel Cells"

Operating Proton Exchange Membrane Fuel Cells (PEMFC) at temperatures above 100^oC is one way to alleviate the problem of the CO poisoning of the Pt anode electrocatalyst. However, the mechanism of proton conduction in the perfluorosulfonated polymer Nafion^TM membrane, commonly used in PEMFC because of its high ionic conductivity under high-humidity conditions, relies on the presence of water in the material. At low humidity and/or elevated temperatures, the water content of the polymer decreases, leading to a decrease of its proton conductivity and consequently of its fuel-cell performance.

To enhance the water retention properties of Nafion^TM at high temperature, incorporation of hydrophilic oxides has been investigated extensively. Sol-gel techniques are particularly suitable, as oxides have been shown to enter the nanopores of the polymer [1]. Our recent work [2] was focused on Nafion^TM membranes infiltrated by a sol-gel process with a phosphosilicate glass 7SiO₂-2P₂O₅-ZrO₂. Phosphosilicate glasses are well known for their fast-proton conductivity, attributed to strong hydrogen bonds between water molecules and the glass P-OH groups. Fuel cell results carried out on this composite showed that PEMFC performance at 130^oC-140^oC was significantly enhanced compared to the original Nafion^TM. This result was attributed to the improved water retention properties of the composite membrane. To understand more accurately the role of the glass in the composite, we are performing impedance spectroscopy measurements at various relative humidity (RH) in the range 25^oC-100^oC on different membranes, namely Nafion^TM, Nafion^TM/7SiO₂-2P₂O₅-ZrO₂, and Nafion^TM/ZrP particles . Up to 130^oC, the composites, except for Nafion^TM/ZrP, exhibit similar conductivity values, typically ranging from ~2.10^-4 S.cm^-1 at 5% RH to ~6.10^-2 S.cm^-1 at 95% RH at 25^oC. The Nafion^TM/ZrP sample shows conductivity values an order of magnitude lower. Conductivity increases slightly with increasing temperature for all the samples. The variation of the complex part of the permittivity with frequency shows that, for all the membranes, long range ion displacement dominates even at low RH. The study of the dielectric loss tangent reveals that the distribution of the water clusters could be different in the composite membranes compared to Nafion. This result should help us to understand the interaction between the polymer and the glass, and lead to improved composite membranes.

[1] K. A. Mauritz, Mat. Sci. Eng., C6, 121 (1998)

[2] M. Aparicio et al, Ceram. Trans. 127 : Materials for Electrochemical Energy Conversion and Storage, ed. S. K. Sundaram, American Ceramic Society, Westerville, OH, 2001.

This page is maintained by Manuel A. Alvarez.
Last updated March 28, 2002.
For any comments or questions please send e-mail to: solgel@rci.rutgers.edu